1. Field of the Invention
The present invention relates generally to passive residual heat removal systems and nuclear power plant equipment. More particularly, the invention is directed to a passive residual heat removal system suitable for application to, and nuclear power plant equipment suitably applicable as, boiling-water reactor equipment.
2. Description of the Related Art
For example, JP-2003-240888-A discloses a primary containment vessel cooling system designed so that in case of fluid line breakage of a nuclear reactor, the system can suppress increases in surface temperature of a suppression pool, and thereby suppress increases in containment vessel pressure.
As shown in FIG. 5, the primary containment vessel cooling system includes a coolant pool 37 filled with a coolant and opened to the atmosphere above a primary containment vessel 42, a heat exchanger 34 placed under the coolant in the coolant pool 37, a steam header 33 and condensate header 35 connected to an upper section and lower section, respectively, of the heat exchanger 34, a steam supply line 32 interconnecting a drywell 38 and the steam header 33, a condensate drain line 53 interconnecting the condensate header 35 and a reactor pressure vessel 31, and a non-condensable gas vent line 44 interconnecting the condensate header 35 and the suppression pool 40. A check valve 54 and a valve 55, both for preventing a backflow of a fluid from the reactor pressure vessel 31 into the condensate header 35, are arranged on the condensate drain line 53. The drywell 38 and the suppression pool 40 are interconnected by a vent line 39, the vent line 39 being open into both of the drywell and the suppression pool. The non-condensate gas vent line 44 is disposed so that height of its open end in an internal fluid of the suppression pool 40 is greater than height of an open end of the vent line 39 in another internal fluid of the suppression pool 40.
In case of fluid line breakage of the reactor pressure vessel 31, steam flows out from the reactor pressure vessel 31 through the broken line into the drywell 38. One part of the steam which flows out in this way is routed into the suppression pool 40 through the vent line 39 and becomes condensed. The remaining part of the steam which flows out is routed into the heat exchanger 34 via the steam supply line 32 and, after heat removal in the coolant pool 37 located outside the heat exchanger 34, becomes a condensate, which is then routed into the suppression pool 40 via the non-condensate gas vent line 44.
Immediately after the line breakage causing a large amount of steam to be released to the drywell 38, much of the steam is guided into the suppression pool 40 via the vent line 39 having a large bore. After this, when the amount of decay heat decreases and thus the amount of steam released also decreases, a pressure loss in the flow channel routed via the heat exchanger 34 will be less than that of the flow channel routed via the vent line 39. As a result, the condensate of the steam which has flown through the heat exchanger 34 and the non-condensate gas vent line 44 will be guided to the suppression pool 40.
In this case, the difference between the flow channel pressure losses on the two routes arises from the fact that the open end of the non-condensate gas vent line 44 in one internal fluid of the suppression pool 40 is disposed at a height greater than the open-end height of the vent line 39 in the other internal fluid of the suppression pool 40.
In addition, the condensate that was condensed in the heat exchanger 34 and will be used as a coolant can be returned to the reactor pressure vessel 31 by opening the valve 54 on the condensate drain line 53 after an internal pressure of the reactor pressure vessel 31 and that of the drywell 38 have become substantially equal.